A. Technical Field
The present invention relates generally to metering technologies, and more particularly, to efficient and accurate methods of calculating root-mean-square values of an alternating current within a metering device.
B. Background of the Invention
The importance of efficiently and accurately measuring current, voltage and power levels of an alternating current (hereinafter, “AC”) signal is well understood by one of skill in the art. Oftentimes, an accurate measurement of certain types of current and voltage levels requires that both low frequency and high frequency components of the AC signal are addressed in measurement or calculation processes. The measurement of high frequency components within the AC signal is oftentimes complex and requires a high speed analog-to-digital converter (hereinafter, “ADC”) to properly sample the high frequency component. The inclusion of such a high speed ADC within a metering system can significantly increase the die size of the system, the power consumption as well as its overall cost. As a result, traditional metering systems usually ignore high frequency components of the AC signal when identifying certain signal characteristics such as root-mean-square (hereinafter, “RMS”) voltage and current levels and, in so doing, sacrifice a level of accuracy in these calculated levels.
In the case of electricity metering devices, an AC signal to be measured may include high frequency harmonics and high frequency switching currents induced by power factor correction circuits commonly employed to increase power efficiencies. Other high frequency components may also be present on the AC signal, all of which complicate an accurate determination of RMS voltage and current levels of the signal.
Traditional metering devices measure voltage and current on the AC signal, and calculate corresponding power and RMS voltage and current levels. These calculations include averaging, filtering and mathematical operations which are usually simple to implement in the digital domain of the metering device using modern digital signal processing techniques. However, the use of these digital signal processing techniques requires that the AC signal be converted into the digital domain, which is typically done by ADCs. In prior art systems, a high speed ADC within the metering device was required in order to allow conversion of the high frequency component of the signal into the digital domain and measurement of both low and high frequency components of the AC signal. The cost of this high speed ADC is usually too high for a commercially feasible metering device so higher frequency components were ignored and certain RMS calculations contained a corresponding error.
FIG. 1 illustrates an exemplary metering device in which high frequency components are ignored in calculating an RMS current of an AC signal. The metering device 110 provides an RMS voltage calculation, an RMS current calculation and a power calculation. The metering device 110 comprises a voltage input on a first ADC 115, which converts the voltage into the digital domain, and a low frequency current input on a second ADC 120, which converts the low frequency current into the digital domain. High frequency current components in the AC signal may be removed from the current input by using a low pass filter (not shown) in front of this low frequency current input. In certain examples, a single ADC is used to convert both the voltage and current inputs into the digital domain.
In calculating the power, the digitized voltage and low frequency current values are multiplied by multiplier 125. A low pass filter 130 averages the output of the multiplier 125 and generates a power reading.
In calculating the RMS voltage, the digitized voltage is squared by squarer 135 and a low pass filter 140 averages the squared voltage. A square-root module 145 performs a square rooting operation on the averaged squared voltage and generates a corresponding RMS voltage reading.
In calculating the RMS current, the digitized low frequency current is squared by squarer 150 and a low pass filter 155 averages the squared current. A square-root module 160 performs a square rooting operation on the averaged squared current and generates a corresponding RMS current reading.
As previously discussed, the failure to include high frequency components within these operations results in error within one or more of these readings. For example, in certain instances the RMS current reading may be inaccurate by as much as twenty percent from the true RMS current of the AC signal because the high frequency current component is disregarded.
There are certain environments in which accuracy in AC signal metering is important such as data centers in which a large number of computing device operate, and power management and heat monitoring is critical in proper functioning. Failure to properly measure or calculate power, RMS voltage and RMS current levels may lead to mismanagement of the devices and possible damage. Accordingly, what is needed is a device and method that is able to efficiently account for high frequency components of an AC signal in metering measurements and calculations.